Cathode-ray tube power supply



R. v. LITTLE, JR., ErAL 2,621,305

CATHODE-RAY TUBE POWER SUPPLY Dec. 9, 1952 Filed Jan. 2, 1951 Wma.naw/1L was@ ,4MM/nia L az//f .4 Evi/v5 BY n E ATTOR'NEY Patented Dec.9, 1952 UNITED y.szrAfrfes ATENT OFFICE CATHODE-,RAY vTUBE POWER SUPPLYfof Delaware ApplicationiJanuary 1951, Serial No. 204,028

13 Claims. 1

'This 'invention Irelates gto 'improvements vin cathrode ray ytube`v-voltage supply systems, -and particularly 5to an improved`:arrangement Afor maintaining `proper focus -of the cathode ray beam."-While not'limited-lthereto,'the present `invention finds :particularapplication to, yand -wiil be .described with special reference to,cathode ray itubeslof the type used-in television systems; so-calledk-inescope tubes.

.In conventional loperation of television Lkinescopes, it has beenAfound lthat :in order to 'maintain'proper` focusfofl ther' cathoderaybeampa 'constant ratio should 'be malntained 4`between lthe voltagesapplied :to @the Ir'st 'zand second anodes. Thus, zforcexample,.;ifrthelsecond .anode :voltage changes say, 5%, then the first anode voltagealso should:.change.:5.% pto maintain ,proper ffocus.Ifxthepower;supplyzuseds reasonablywell regulated, athesevoltagefrelationships are notzdiicult to maintain under normal...operating conditions.

However, when the lkinescope is operated :at very high Acurrent levels,it .has been foundethat defocussing .may occur even though `-the ratiobetween the anode voltages iskept constant. For example, inthe case ofprojection-type rtelevision kinescopes, ,utilizing comparatively `:highbeam currents, it has been `found-that 7.thelcathode .ray beam tends tozdeiocus, particularly during intervals when the scene ...beingtelevised has a high brightness level.

While thereason for this .phenomenonis not entirely understood, v it isthought to be related... in part, -to .the fact .that the diameterof thecathode raybeam'tendsto.increase as the beam density increases. .ThisYchange Ain .diameter apparently requires a .different .relation betweenthe anode voltages, in order tomaintain the Vbeam focus, than isYrequired Vatlower -current levels. Onthe otherlhand, ifthe supplyvoltage changes, it'is necessary to maintain this new anode voltageratio which is required at vhigh current levels, just as it is necessaryto maintain the proper ratiov at'lower current levels.

'It'is,therefore, a'generalobject ofthe present inventionto provide animproved'cathode ray tube power supply system adapted Vtto -prevent beamdefocussing notwithstanding changes in the cathoderaytube current.

Another object of the invention'is to provide an improved high Voltagecontrol'circuit for a cathode ray tube.

In `accordance with theinvention, the foregoing and other relatedobjects are attained' by provision of a controll circuit which reducesthe rst anodevoltageof-.the cathode raytube when the tubecurrerit'increases,l as it hasbeen-foundlthat such change in first anode voltagewill jprevent defocussing of the cathode ray when the tube currentchanges.

A -more complete understanding of the Iinvention can -be yhad byreference to the following description of ilustrative embodimentsthereof, when considered in connection with lthe =Yaccom panyingdrawing, wherein:

`Fig. 1 is afschematic diagram of a television kinescope power supplysystem arranged in-accordance with the invention, Aand Fig. 2 Ais faAschematic Idiagram "of fa Vfmodiiied form of televisionkinescope powerysupply `system further illustrating the .principlesof the invention.

VReferring'to Fig. 1,:there vis shown a television kinescope .tube Ill,Ahaving .a cathode .4.2, :.a'irst anode l, andlasecond. anodeLit,thellatterucomprising .a conductive v.coating .on the @inner tubesurfaceadjacent the .screen endrthereof. '.Other .elements of .thel tube`I Bare not-shown in'Figal as Athey are not .necessary to anunderstanding of vthe .present invention. As :is .Well known, fthe rstand second anodes lil, ,flaare ,cooperableto focus (at the vscreenendeof the tube l0.) the cathode ray ,providediby current emitted fromthe .cathode l2.

Energizing -voltagesfor .theitube Lil-arexprovided .by .a qpower `supplysystem t8, illustratively shown as comprisingtaso-calledvoltageidol-lbler circuit. Thiscircuit t8 includes a transformer 2lithrough which alternatingvoltage canghesupplied, from any suitablesource (notfshown), :,to chargeapair of capacitors 22-24 throughrectiers 26, 28. Since the-,powersupplylgenerally is conventional, a.detailed ,Y description .thereof will not be given. ASulilceto.statethat the` rectifiers 26, 28 lwllvoperatefto storeon the capacitors 22,24 unidirectional voltages Asubstantially equal to thepeaktransformer.output-voltagaso that the sum of the capacitor .voltages`willgloe substantially twice .the peak .output ^voltage .of thetransformer. Eor example, l.thefsilin-of the capacitor voltages maylbeof the 4orolerofeighty kilovolts, the voltage acrosseachlbeingapproximately forty Y kilovolts.

The second anodel of thekinescope tube "l0 is connected'to a highVoltage output terminal 25 to -receive the `full output 'voltage of theipower supplyl, while the rst anode lfllis connectedto an intermediatevoltage output terminal '21. AIn the specic povversupplyls showninFig.j1, the terminal `2l comprises the junction oa-pair of voltage dividerresistors-30, 32. "Thevoltagedivider resistors vtil', V32 zareconnectedfacross lathe capacitor l22 and suitably .proportioned vf to-:give

the proper nominal ratio of second anode to first anode voltage. In atypical case, the nominal value of first anode voltage may be of theorder of 20 kilovolts.

If the tube lll were to be operated only at comparatively low currentlevels, the system thus far described normally would functionsatisfactorily. Assuming that the first anode I4 does not draw anyappreciable amount of current, the cathode ray beam in the tube l couldbe expected to stay reasonably well focussed. The resistor 32 mightcomprise a potentiometer having a tap connected to the first anode I4 topermit nominal adjustment of the anode voltages ratio.

However, as previously explained, if the tube l0 is operated at highcurrent level, it has been found that the beam tends to becomedefocussed even if the ratio between the anode voltages is keptconstant. It has also been found that this defocussing effect can beovercome if the rst anode voltage is reduced as the beam currentincreases.

Accordingly, in accordance with the invention, there is provided acontrol circuit which is connected to cause a decrease in the firstanode voltage, by increasing the voltage drop across the resistor 3|),when the beam current increases. This control circuit includes a vacuumtube 34 connected in shunt with the lower voltage divloer resistor 32.

Since the first anode voltage ordinarily will be considerably greaterthan the maximum permissible operating voltage of an ordinary tube, thecontrol tube 34 preferably comprises a modified kinescope tube.Satisfactory results have been obtained by replacing the usual secondanode coating in such tube with a high dissipation metal anode disposedin the beam path. The other elements of the electron gun in the tube canbe utilized in their conventional form. It has been found that with thesecond anode modification just referred to, the kinescope tube willfunction in a manner very analogous to an ordinary pentode vacuum tube.Therefore, the tube 34 has been shown schematically as a pentode tube,having a cathode 36, a control grid 38, a screen grid 40, a suppressorgrid 42 and an anode 44. structurally, these electrodes 36-44 comprisethe cathode, first and second grids, and rst and (modified) secondanodes, respectively, of the modified kinescope tube. For convenience,the pentode terminology will be used hereinafter.

The anode 44 of the tube 34 is connected to the intermediate voltageoutput terminal 21 in common with the first anode I4 of the tube I0. Thescreen and suppressor grids 40, 42 of the tube 34 are connected toadditional taps 29, 3| on the lower voltage divider resistor 32. Thecontrol grid 38 is connected to a fixed source of negative bias voltage,shown as a battery 46.

The cathode 36 of the tube 34 is connected to the adjustable tap 48a, ofa potentiometer 48. This potentiometer 48 is connected in the negativereturn lead of the power supply I8, and is shunted by a capacitor 50.Therefore, it can be seen that the voltage at the potentiometer tap 48awill be a negative voltage (with respect to ground), of amplitudedetermined by the cathode ray tube current. Accordingly, the controlgrid-to-cathode voltage of the tube 34 will be determined by the cathoderay tube current.

The voltage drop across the resistor 30 will be a .function of thecurrent drawn by the control tube 34. In turn, the current drawn by the4 tube 34 will be a function of the control grid-tocathode voltagethereof. Since the control grid 3B is connected to a fixed voltagepoint, the factor determinative of the control tube current will be thevariable cathode voltage thereof.

The anode voltages of the tube I0 rst are adjusted (by setting thepotentiometer tap 48a) so that the cathode ray beam in the tube l0 isproperly focussed at a low value of beam current. Thereafter, if thebeam current increases, the voltage at the potentiometer tap 48a willbecome more negative. This will effectively make the control grid 38less negative with respect to the cathode 36, causing the currentthrough the control tube 34 to increase. In turn, the voltage dropacross the resistor 30 will increase, reducing the first anode voltageof the tube I0 sufciently to maintain proper focus of the beam therein.

Throughout the foregoing discussion, it has been assumed that the powersupply output voltage remains essentially constant, and that the rstanode voltage only is changed to provide a new ratio of cathode ray tubeanode voltages when the beam current changes. If the power supply I8 isnot capable of maintaining essentially constant output voltage withinthe limits of system operation, an additional problem arises.

The control tube 34 has the usual "constant current characteristic of apentode vacuum tube. That is, for a given value of controlgrid-tocathode voltage, the tube current will have a relatively constantva1ue for relatively wide variations in anode voltage, provided thescreen and suppressor grid voltages are kept essentially constant.

Therefore, it can be seen that a change in the power supply outputvoltage might cause defocussing of the beam in the tube I0. Since thecurrent through the control tube 34 can be substantially independent ofanode voltage changes, a change in the power supply output voltage maynot cause any change in the voltage drop across the resistor 30. If thisoccurs, there would be a constant difference between the second andfirst anode voltage with relatively large changes in power supplyoutput, rather than the constant ratio that is needed to maintain focus.Assume, for example, that the nominal second anode voltage is kilovolts,and the nominal first anode voltage 20 kilovolts. Assume, further, thatthe power supply output changes so that the second anode voltage drops2.5 per cent to 78 kilovolts. The proper corresponding change in firstanode voltage would be a drop to 19.5 kilovolts. With a constant voltagemaintained across the resistor 30, however, the first anode voltage maydrop to, say, 19 kilovolts. This would cause defocussing of the beam inthe tube I ll regardless of any effect dueto beam current changes.

Where the power supply output voltage is not subject to much variation,the difficulty arising from the constant current characteristic of thecontrol tube 34 will be adequately compensated by connecting the screenand suppressor grids 40, 42 to the output terminals 29, 3 i. The changesin voltage across the resistor 32 which will result from changes inpower supply output voltage will change the current through the controltube 34 suiciently rtomaintain the ratio of the anode voltages nearlyconstant. However, if it is found that these changes in the screen andsuppressor grid voltages do not provide adequate compensation, thearrangement shown in Fig. 2

maar;

da flue'atdlorllted. JAlscfs hown ri1rFi`gr-2 isfan-alternative sourceor 4cfontfrfl -grid-v-'tocathode voltage for4 thejcontrol tube '3o. I

l the-'system shown inFig. 2, the control grid voltage for vthe controltube 34 comprises the videoisignal 'applied to the ki'nescope controlgrid L5.1 Since the beam'current inthekinescope tube I-dwillfbe ydetermined by the'video signal *applied to the jcontrolf'grid thereof,the video Isignal will bejrrepresentative of the "kinescope beamcurrent., AAccordingly,"the "control grid 38 of 'the control tube 'denisIconnected in common with the `-kinescope control lgrid i5 `to l"theoutput `of the D C. resto'rer circuit r52 of `thevideo signal network. ul

"'Afs the 'videosignal changes, `carts'ing 'changesin"then'lginescopejcurrent, it Acan be seen that theourrent through the4control tube 315 will change correspondingly to raise and Vlower thefirst V"a'rrio'de voltage of the kinescope as the beam current increasesand decreases.

'In Fig. @2, the power supply il 8 is shown'in block forin, 'andisdesignated asa high impedance power .supply to indicate that theinternal impedance thereof is sufficiently high tc cause an appreciabledecrease in output voltage when the outputourrent increases. It will beunderstood thai-, the internal impedance of the vsupply i8 willdepnd'onthesizes'and ratingsfof the transf ormers, capacitors, and otherelements used therein.

Witha high impedance power supply, itis necessary toprovide some meansfor maintaining the ratio of 'anode voltages at the proper value sincethe power supply output voltage usually will change as the load currentchanges. In the system of Fig. 2, the screen grid t in the control tubeSll is. connected to the vtap 59a of a potentiometer which is connectedin series with a voltage regulator element 62 across the second `anodecircuit of the kinescope to. The regulator 'element 62 may comprise aso-called corona regulator, or any equivalent element which'has thecharacteristic of conducting diiferent 'amounts of current at constantvoltage drop. The regulator 'element e2 will transfer substantially 'allof any'second anode voltage change to the lpoteritiometeri, therebyproviding a large compensating voltage for the control tube screen grid40. The sequence of operations in Fig. 2 may be somewhat as follows:

A decrease in the output voltage of the power supply I8 would cause adecrease in the voltage at the high voltage terminal 25. The voltage atthe intermediate output terminal 21 also would decrease. Due to theconstant current characteristic of the control tube 3d, the change inoutput voltage at the intermediate terminal 21 would tend to be greaterthan it should be to maintain the proper ratio of cathode ray tube anodevoltages. However, the Voltage at the control tube screen grid 4E] willdrop as the second anode voltage decreases, thereby decreasing thecurrent through the control tube 34. This will decrease the internalvoltage losses in the circuit supplying voltage to the output terminal21, thereby causing the rst anode voltage change to be more nearlyproportional to the second anode voltage change. To avoid confusion, itshould be noted that the compensating eiect provided by the screen grid40 :is distinct from the effect provided by changes in the control gridvoltage due to changes in the Video signal. It should also be noted thatthe screen grid compensation circuit ordinarily will not be required ifthe 6 control tube 34 does not have 4v'a constant'currentcharacteristic.

The correct nominal ratio between second and rst anode voltage in Fig. 2can be established by adjusting the potentiometer tap toa.

One of the advantages of the circuits of Figs. 1 and 2 not previouslymentioned resides inthe fact that no high voltage potentiometers arerequired for adjusting the focus of the -kinescope beam. As is wellknown, po-tentiometrs designed to handle voltages of the order of twentyor more kilovolts are diicult to construct,'rela tively costly, and alikely source of operational diliculties.

From the foregoing, it will be apparent that the present inventionprovides an eicient dual purpose control circuit for a kinescope Vpowersupply system, compensating for deiccussing eiects due both to changesin kinescopecurrent and changes in power supply output voltage.

What is claimed is: n

1. In a voltage supply system for a cathode ray tube of the type havinga rst and a second anode cooperable to focus the cathode ray beamprovided by current flow through said tube, in combination, a voltagesource, a first voltage supply circuit connecting said rst anode to saidsource to supply to said rst anode unidirectional voltage or magnitudeinversely proportional to the current drawn through said first circuit,a second voltage supply 'circuit connecting said second anode to saidsource to supply unidirectional voltage to said second anode, and athird circuit connected to said rst circuit and comprising means toincrease the current drawn through said nrst circuit in response to anYinrease 'in the current ilowing through said tube.

2. rihe combination deined in claim i wherein said means in said thirdcircuit includes `a ypotentiometer common to said first andsecond'circuits for adjusting the ratio betweenthe voltages applied bysaid first and second circuits vto said rst and second anodes.

3. The combination dened in claim 1 wherein said means in said thirdcircuit includes an electron'tube having an anode, a cathode and acontrol grid, and means to apply between said control grid and saidcathode a voltage proportional to said cathode ray tube current.

4. The combination dened in claim '3 wherein said last-mentioned meanscomprises a potentiometer common to said first and second circuits andhaving an adjustable tap, said cathode being connected to said tap.

5. In a Voltage supply system for a cathode ray tube of the type havinga rlrst and a second anode cooperable to focus the cathode ray beamprovided by current iow through said tube and having a control electrodeto which signals are applied to control said current, in combination, avoltage source, circuits connecting said anodes to said source to supplyto said anodes unidirectional voltages of different magnitudes, anelectron tube included in the circuit connecting said rst anode to saidvoltage source, impedance means in series with said electron tube insaid first anode circuit providing with said electron tube a voltagedivider, an intermediate point on said voltage divider to which said rstanode is connected, said electron tube having a control grid connectedto receive signals applied to said cathode ray tube control grid.

6. The combination defined in claim 5 wherein said electron tube isprovided with an additional current control electrode connected to saidsecond anode circuit to receive a voltage proportional to that appliedto said rst anode.

7. 'Ihe combination dened in claim 6 wherein said connection from saidsecond anode circuit to said additional electrode comprises a voltagedivider network having as an element thereof a voltage regulator.

8. In a cathode ray tube system comprising: a cathode ray tube havingfirst and second anodes, a unidirectional voltage source having firstand second voltage output terminals, and circuits connesting said iirstand second anodes to separate ones of said output terminals to drawcurrent through said cathode ray tube; the combination with said systemof impedance means in said rst anode circuit to decrease the voltageapplied to said first anode upon an increase in current flow throughsaid rst circuit, a vacuum tube having an anode, a cathode and a controlgrid, a circuit connecting said vacuum tube to said rst anode circuit topass current through said impedance means in an amount determined by thevoltage applied between said control grid and said cathode, a circuitapplying to said control grid a voltage negative with respect to saidcathode, and means connected in said last named circuit to decrease saidlast named voltage in response to an increase in the current throughsaid cathode ray tube.

9. The ycombination dened in claim 8 wherein said means in said lastnamed circuit comprises an impedance common to said first and saidsecond anode circuits.

l0. The combination dened in claim 8 wherein said vacuum tube isprovided with an addtional current control electrode, said additionalcontrol electrode being connected to receive from said voltage :source avoltage proportional to the voltage applied to said second anode.

11. The combination dened in claim 8 wherein said impedance meanscomprises a resistor connected between said voltage source and said rstanode.

12. In a cathode ray tube system comprising: a cathode ray tube havingrst and second anodes, a unidirectional voltage source, and circuits toapply to said anodes from said source voltages of diierent predeterminedmagnitudes; a control network to decrease said voltage applied to saidfirst anode in response to an increase in the current owing through saidtube, said control circuit comprising impedanoe means in the circuitapplying said voltage to said first anode to reduce the voltage appliedto said rst anode in response to an increase in the current through saidimpedance means, a circuit connected to pass current through saidimpedance means, a vacuum tube connected in said last named circuit,said vacuum tube having electrodes including a control grid and acathode, and means to apply between said control grid and said cathode avoltage of magnitude proportional to the current owng through saidcathode ray tube as a control of the current through said vacuum tubeand said impedance means.

13. The combination of a cathode ray tube having rst and second anodes,a unidirectional voltage source having high and intermediate voltageoutput terminals, a circuit connecting said ilrst anode to saidintermediate voltage output terminal, means in said circuit fordecreasing the voltage applied to said rst anode in response to anincrease in current ow through said circuit, a vacuum tube connected inparallel with said circuit, said vacuum tube having an anode, a cathodeand a control grid, said vacuum tube anode being connected to saidcathode ray tube rst anode, means to apply voltage between said controlgrid and said cathode, and means to control the grid-to-cathode voltageof said vacuum tube as a function of the beam current in said cathoderay tube.

RALPH VINTON LITTLE, Jn. LOUIS LONESS EVANS.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,173,221 Ballard Sept. 19, 19392,218,720 Rinia Oct. 22, 1940 2,291,682 Blumlein et al Aug. 4, 19422,302,876 Mailing Nov. 24, 1942 2,371,897 Knick Mar. 20, 1945 2,434,196Cawein Jan. 6, 1948 2,459,602 Ulman Jan. 18, 1949 2,555,147 Meagher May29, 1951 2,567,377 Holbrook Sept. 11, 1951

